Apple has just published the latest upgrade to Safari,version 5.0.1, which includes a whole new feature called “Extensions Gallery,” as well as a number of other interesting fixes.
The Extensions Gallery is essentially a center for add-on features and toolbars for a variety of different web programs and services
All of them are built using HTML5, CSS or Javascript technologies. You can get a full look at the Extensions function before downloading on Apple’s site.
Source: ZDNet Community
So far you've only be able to drag in attachments and images into email messages in Chrome and Firefox. Now, this feature work in the newest version of Safari (Safari 5) too.